Surface acoustic wave devices have won a niche in the electronic signal processing art. They are especially common as intermediate frequency bandpass filters in the I.F. sections of television receivers. These devices comprise an input interdigital transducer and an output interdigital transducer on a common substrate. The input transducer receives an electrical signal and converts it to an acoustic wave which is transmitted over the substrate to the output transducer. The output transducer then reconverts the acoustic wave to an electrical output. The frequency response of both transducers is a passband centered on the I.F. of the T.V. receiver.
A problem which has been encountered with such filters is caused by acoustic echoes. The acoustic signal generated by the input transducer is not fully converted into electrical energy by the output transducer. Some of this acoustic energy is reflected from the output transducer back to the input transducer (first reflection). Then a portion of the first reflection is re-reflected from the input transducer back to the output transducer (second reflection). An acoustic wave which has made three one-way trips (input to output, back to input, and back to output again) is called the triple transit reflection. Because of the transit time required, the reflection is delayed relative to the original signal. Consequently, when S.W.I.F. devices are used as I.F. filters in T.V. receivers, the triple transit reflection can cause an unacceptable ghost image on the picture tube.
This is particularly true when it is desired to reduce the insertion loss of the filter. Two options are available to accomplish this objective: use of a filter substrate material with a higher coupling factor; and reduction of matching losses by transducer tuning (i.e., adding an external inductance to balance the capacitive reactance of the transducer at the signal frequency). Both of these approaches have the unfortunate side effect of increasing echo amplitudes, often to unacceptable levels. Thus, where low insertion loss is a requirement, triple transit reflections can cause difficulty.
A significant body of prior art is devoted to the solution of this problem. Most of this art is directed at mitigating or overcoming the effects of the first reflection (from output transducer to input transducer). Few attempts appear to have been made to mitigate or overcome the effects of the second reflection (from input transducer to output transducer) or, better yet, of both reflections.
DeVries U.S. Pat. No. 3,727,155, Deacon U.S. Pat. No. 3,984,791, and Hunsigner et al. U.S. Pat. No. 4,162,465, recognize that when the interdigitated fingers of the input and/or output transducers are bifurcated and both halves thereof are connected to the same bus bar ("split-connected") then reflection amplitudes can be substantially reduced. When split-connected fingers are used on both the input and output transducers, as in the DeVries and Deacon patents, then both the first and second reflection amplitudes can be reduced. This phenomenon, however, depends heavily upon the external impedances connected across the transducers, and therefore is not a general solution to the problem.
Gilchrist et al. U.S. Pat. No. 4,205,280 employs an electrically isolated transducer finger to ameliorate reflection problems. The theory of operation of the Gilchrist et al. device is that reflections from the isolated finger will be in phase opposition to reflections from other fingers, and thus cause some degree of phase cancellation.
Various other prior art approaches also depend upon phase cancellation phenomena. Coussot U.S. Pat. No. 4,025,880 employs a metallic wafer for phase shifting, while Dempsey et al. U.S. Pat. No. 4,146,851 makes use of a special reflector plus phase shifts introduced by a multistrip track coupler.
A number of prior art devices employ an extra transducer to create out-of-phase effects which partially cancel the unwanted reflection. In Hunsigner U.S. Pat. No. 3,908,137 there is an extra input transducer, electrically delayed relative to the primary input transducer. In Dias U.S. Pat. No. 3,596,211, Knowles U.S. Pat. No. 3,626,309, and Jones U.S. Pat. No. 3,810,257 there is an extra output transducer, shifted along the acoustic beam axis by a fraction of a wavelength relative to the primary output transducer. In the Dias patent the extra output transducer is a "dummy," i.e., it is not electrically connected to the output circuit of the primary output transducer. The present invention is an improvement upon that type of device.
Phase cancellation schemes which depend solely upon displacements along the acoustic beam axis generally suffer from bandwidth limitations. A surface acoustic transducer can be designed to have a relatively narrow or broad bandwidth, by proper choice of the spacing between interdigitated pairs of transducer elements. Thus the effective bandwidth of many triple transit phase cancellation schemes can be narrower than the passbands of the transducers with which they are used.
Moreover, phase opposition alone is insufficient to guarantee total or even near-total cancellation of unwanted echoes, unless the amplitudes of the opposed reflections are equalized.
This invention attempts to control both phase and amplitude, to achieve triple transit echo cancellation.
Moreover, the invention attempts to control phase and amplitude relationships over the entire bandwidth of the device.
The invention recognizes that the echo of a dummy transducer must not only be in phase opposition to the unwanted reflection, but that it must also achieve amplitude equality so that the goal of total cancellation can be reached. The choice of an offset along the acoustic beam axis is not sufficient in itself to guarantee total cancellation, even at the center frequency of the passband. A phasor expression which specifies the amplitude and phase of an acoustic echo under all conditions is a function not only of position along the acoustic beam axis, but also of the number of interdigitated transducer elements and their interdigitated lengths.
In addition the phasor is also a function of such electrical parameters as the admittances of the transducer itself and of any external or parasitic impedances across it, including short-circuit (zero impedance) and open circuit (high impedance) conditions. Moreover, these factors must be taken into account not only for the primary transducer (in order to calculate the general phasor expression for its reflection under any operating conditions) but also for the dummy transducer (in order to calculate the same phasor for the dummy) and thus guarantee phase opposition and equality of amplitude between the two phasor quantities.
When this unprecedented level of sophistication in design is employed, the reflections from the primary and dummy output transducers not only achieve full cancellation, but the results are valid over the entire bandwidth of the transducers employed because the admittances employed in its calculations are frequency-dependent variables.
Cancellation of the second reflection is also a possibility. The invention contemplates dividing the input transducer into primary and dummy sections, and arranging the dummy input transducer so that its second reflection cancels the second reflection from the primary input transducer. Furthermore the input and output transducer system may both be divided into primary and dummy sections, and the dummy sections arranged for cancellation of both reflections. Preferably a compensated input transducer system of this kind is designed according to the same mathematically sophisticated phasor cancellation criteria described above.
Additionally, the invention contemplates the use of apodization, or finger length weighting, to improve the frequency response characteristics of a transducer system which comprises both a primary and a dummy transducer.
The foregoing features of the invention, as well as others, will be more fully appreciated from a reading of the detailed description of the preferred embodiments which appears below, when read in conjunction with the following drawings.